build: curvetun: add NACL_LIB variable to configure to be able to set an alternative lib

This way one can simply set an alternative lib like libsodium for
linking the curvetun binary.

The current default linking against libnacl is preserved!

To link curvetun against libsodium you can set NACL_LIB=sodium

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
doc: curvetun: add documentation how to use libsodium instead of libnacl

Add information to INSTALL on how to build with libsodium.

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>